The present invention is in a process for cooling a hot process gas in which the gas is fed through a stationary fluidized bed, which contains cooling elements, part of the solids suspended in the gas stream are separated in the dust-containing space over the fluidized bed and are recycled to the fluidized bed. The solids separated from the exhaust gas in a deduster are recycled to the fluidized bed.
In some processes, a hot process gas is formed which can be cooled only with considerable difficulty. For instance, a process gas may contain condensable components or entrained liquid droplets, e.g., of metal or slag. Such condensable components or entrained liquid droplets may form crusts on cooling surfaces when the gas is cooled. The process gas may contain poorly flowing fine dusts, which may form crusts even at the temperature of the process gas or when cooled. The process gases may also contain SO.sub.3, or SO.sub.3 may be formed in response to a cooling, or an undesired sulfatizing may occur.
German Patent Specification 34 39 600 discloses that a process gas formed by the gasification of carbonaceous solids can be cooled by supplying the hot process gas to, and cooling the gas in, a stationary fluidized bed of sulfur-binding solids. The fluidized bed contains cooling elements through which a cooling fluid passes. The fluidizing gas consists of a recycled partial stream of the process gas exhausted from the fluidized bed. The process gas is introduced into the fluidized bed from the side or from above. The cooled process gas, which has left the fluidized bed, is dedusted in a cyclone, cooled further in a heat exchanger, and introduced into a gas purifier. The solids removed in the cyclone and in the gas purifier are recycled to the fluidized bed. This procedure does not avoid contact between the process gas and cooling surfaces so that crusts may be formed. An optimum mixing of the process gas and solids is not achieved.
U.S. Pat. No. 3,977,846 discloses that a process gas, which contains hydrocarbons, can be cooled in a stationary fluidized bed which, in its lower portion, contains cooling surfaces through which a cooling fluid passes. The fluidizing gas consists of an extraneous gas which is free of hydrocarbons. The process gas is introduced above the cooling surfaces through nozzles, which are disposed in the fluidized bed. The nozzles are heat-insulated to prevent the formation of deposits. The cooled process gas leaving the fluidized bed is supplied to a deduster. Solids laden with condensed hydrocarbons are withdrawn from the fluidized bed and fresh solids are charged into the fluidized bed. One must expect a clogging and high wear of the nozzles by the corrosive components and solids contained in the process gas.
U.S. Pat. No. 4,120,668 discloses that a process gas which contains molten salt particles and volatile components can be cooled in a stationary fluidized bed, into which the process gas is introduced as a fluidizing gas. The fluidized bed contains cooling surfaces above the level at which the process gas is introduced. The cooled gas is dedusted in a cyclone, and the removed solids are recycled to the fluidized bed. Part of the solids are downwardly removed from the fluidized bed, and fresh solids are charged into the fluidized bed. In that case the above-mentioned disadvantages will also be encountered.
WO 88/08741 refers to a process wherein a process gas is cooled in a circulating fluidized bed. In that technique the process gas is cooled in a mixing chamber with cooled process gas which is recirculated and with cooled solids which are recirculated. The bottom of the mixing chamber is conical and has an opening for receiving the process gas and the recirculated gas. The suspension leaving the mixing chamber can be cooled further on cooling surfaces in the upper portion of the bed vessel and the solids may subsequently be removed in cyclones and be recycled to the bed vessel. A partial stream of the gas may be recirculated to the bed vessel. Alternatively, the suspension may be discharged without further cooling and the solids may be removed in cyclones and be recycled to the bed vessel, whereafter the gas may be cooled and may be partially recirculated to the vessel. The density of the suspension in the circulating fluidized bed is maintained at 1 to 5 kg/m.sup.3 or lower values and the solids are recycled at a rate of 0.92 to 11.5 kg/sm.sup.3 (sm.sup.3 = standard cubic meter). The large volume of the exhaust gas is due to the high rate of gas recycle and requires an expensive gas purifier. A relatively large heat exchange surface area is required due to the low density of the suspension.